An important need exists to dry grain quickly and effectively after harvest to retain maximum quality, to attain a moisture content sufficiently low to minimize infestation by insects and microorganisms (e.g., bacteria, fungi, etc.), to prevent germination and to maximize consumer acceptability of appearance and other organoleptic properties.
Grains are hydroscopic and will lose or gain moisture until equilibrium is reached with the surrounding air. Grains will dry until they reach their equilibrium moisture content (EMC). The EMC is dependent on the relative humidity and the temperature of the air. The relationship between EMC, relative humidity and temperature for many grains has been modeled by researchers: the results have been summarized in Brooker et al. (1974), Drying Cereal Grains, Westport: The Avi Publishing Company, Inc., 265 pp. For instance, EMC's for certain grains are shown in the chart immediately below.
Relative Humidity (%)30405060708090100GrainEquilibrium Moisture Content (% wb*) at 25° C.Barley8.59.710.812.113.515.819.526.3Shelled Maize8.39.811.212.914.015.619.623.8Paddy7.99.410.812.213.414.816.7—Milled Rice9.010.311.512.612.815.418.123.6Sorghum8.69.811.012.011815.818.821.9Wheat8.69.710.911.913.615.719.725.6*wet basisSource: Brooker et al. (1974)
There are two basic mechanisms involved in the drying process: the migration of moisture from the interior of an individual grain to the surface and the evaporation of moisture from the surface to the surrounding air. The rate of drying is determined by the moisture content and the temperature of the grain and the temperature, the relative humidity and the velocity of the air in contact with the grain. In general, higher airflow rates, higher air temperatures and lower relative humidities increase drying speed. The rate of moisture movement from high moisture grain to low relative humidity air is rapid. However, the moisture movement from wet grain to moist air may be very small or nonexistent. Also, higher airflow rates generally result in higher drying rates.
Traditionally, grain crops were harvested during a dry period or season and simple drying methods such as sun drying were used. However, maturity of the crop does not always coincide with a suitably dry period. Furthermore, the introduction of high-yielding varieties, irrigation, and improved farming practices has led to the need for alternative drying practices to cope with the increased production, and grain harvested during the wet season as a result of multi-cropping.
Among other techniques, in-line dryers have been used for drying the grain. However, these use high amounts of fuel and the dryers act like an oven and tend to cook out all of the moisture and over dry and crack the grain. As a result, it has become common for grain to be stored in bins and dried by mechanically moving air over and through the grain. This method is referred to as the “in-bin natural air drying” technique.
The in-bin natural air drying technique has several advantages. It can increase the quality of the harvested grain by reducing crop exposure to weather and reduce harvesting losses, including head shattering and cracked kernels. It also reduces the dependency on weather conditions for harvest and allows more time for post-harvest field work.
However, current in-bin natural air drying systems have several disadvantages. Grains can only be stored without significant deterioration for a period of time depending on the storage conditions, such as temperature and relative humidity. Thus, the EMC must be attainable within that period of time and thereafter maintainable. Drying fans are costly to operate: they should operate when the relative humidity level is low and temperature levels are generally warm. For instance, it is useless to run fans if it is raining. Also, hot spots, i.e., grain degradation, in the grain are difficult to prevent. Sensors for determining the condition of the grain placed throughout the bin help prevent hot spots. Also, it is preferable for the drying system to be centrally controlled, with remote access.